Kinetic study of n-heptane conversion on sulfated zirconia-supported platinum catalyst:: the metal-proton adduct is the active site

Isomerization and cracking reactions of n-heptane on 0.2 wt.% platinum supported on sulfated zirconia (Pt/SZ) are investigated in such a way to get maximum kinetic data. The experimental conditions were varied as follows: (i) two ranges of hydrogen pressure, low pressure (LP) 190-760 Torr and high pressure (HP) 900-3000 Torr were studied; (ii) reaction temperatures of 150, 200 and 250 degreesC were investigated; and (iii) hydrocarbon partial pressures from 5 to 40 Torr were analyzed. Reaction orders versus hydrogen pressure and hydrocarbon pressure, and apparent activation energy values have been determined. The Pt/SZ catalyst displays an original catalytic behavior: (i) at LP, the hydrogen reaction order is negative, while it is positive at HP, and in both cases, these orders tend towards zero when the reaction temperature increases (ii) the apparent activation energy values are lower at HP than at LP; and (iii) the reaction orders versus hydrocarbon are equal to I whatever the experimental conditions are. The changes in the experimental conditions involve modifications of the kinetic parameters. Such results are explained by the fact that the relative proportions of metallic sites or acid sites are influenced by the experimental conditions, The metal-proton adduct site [H-(M-m)(H+)(x)](x+) explains these experimental results. We interpret that at hydrogen pressures higher than 760 Torr, the excess of hydrogen provokes a shift in the reactivity from the metallic part of the adduct to the acid one. In fact, at HP and at high reaction temperature, the reactivity is controlled by the acid sites, while at LP and low temperature, it is managed by the metallic and the acid sites. (C) 2002 Elsevier Science B.V. All rights reserved.